Tears are a complex mixture of water, lipids, mucus, proteins and electrolytes and this mixture helps to maintain a smooth and clear optical surface and also helps to protect the eyes from infection. The tear film has three basic layers: oil, water, and mucus and problems or disturbances in any of these layers can cause dry eyes symptoms.
The outermost layer of the tear film is typically comprised of an oil layer containing fatty acids and lipids (meibum) which are produced primarily by sebaceous glands called the meibomian glands located along the eyelid margin. The oil layer smoothes the tear surface and slows evaporation of the watery middle layer. However, if the meibomian glands fail to produce enough oil, produce suboptimal fatty acid mixtures, or if the glands become obstructed or clogged, the watery layer typically evaporates too quickly causing dry eyes. A blockage of the meibomian glands can lead to enlarged glands or infections. Dry eyes are thus common in people whose meibomian glands are obstructed or functioning improperly.
The middle layer of tears is composed primarily of an aqueous solution, which is produced by the lacrimal glands (tear glands). The middle layer cleanses the eyes and washes away foreign particles or irritants, maintains a clear optical medium, and keeps the ocular surface moist. The innermost layer of the tear film is composed primarily of mucus which helps to spread the tears evenly over the surface of the eyes. A lack of mucus in the tear film is also associated with dry eye syndrome.
As discussed above, the meibomian glands are oil-secreting glands located within both the upper and lower eyelids. There are approximately 30 to 40 glands along the upper eyelid and approximately 20 to 30 glands along the lower eyelid with the ducts for each of the glands opening along the inner edge of the free margin of the respective lids by minute foramina through which their secretion is released to prevent the lids adhering to each other. An example of the location of the meibomian glands is illustrated in the cross-sectional view of the upper eyelid UL shown in FIG. 1A which illustrates the relative positioning of a single meibomian gland MG. Other glands and anatomical features are illustrated for reference, e.g., the glands of Wolfring GW, tarsus TR, gland of Moll GM, gland of Zeis GZ, gland of Krause GK, upper formix UF, conjunctiva CN and cornea CR of the eye which is partially covered by the upper eyelid UL. As illustrated, the meibomian gland MG is positioned along a length of the upper eyelid UL (and lower eyelid LL) with the duct opening along the inner edge of the eyelid UL in proximity to a surface of the underlying eye.
FIG. 1B illustrates a front view of a patient's eye having the upper eyelid UL and lower eyelid LL in a closed position, such as when the patient blinks. As shown, the meibomian glands MG may be seen aligned adjacent to one another over both the upper UL and lower eyelids LL. FIG. 1C also shows a perspective view of a patient's eye in the open position to illustrate how the meibomian glands are typically aligned relative to one another when the patient's eye is opened.
Blinking is thought to be the primary mechanism to open the orifice of the meibomian glands to allow for the release of oil secretions from the glands. The natural blinking motion and blinking force causes the upper lid to pull a sheet of the lipids secreted by the meibomian glands over the two underlying layers of the tear film thus forming the protective coating which limits the rate at which the underlying layers evaporate. It is estimated that approximately 65% of meibomian gland disease or dry eye results from a defective lipid layer or an insufficient quantity of such lipids that results in accelerated evaporation of the aqueous layer. Hence, blinking disorders, or other disorders that affect proper tear distribution, may also cause or exacerbate meibomian gland dysfunction or dry eye.
As the eyelids close in a total blink, the superior and inferior formices, which hold a reservoir of tears, are compressed by the force of the preseptal muscles and the eyelids move toward one another. The upper eyelid, for instance, moves over the eye while exerting upon the eye surface a force which helps to clear the front of the eye of debris, insoluble mucin, and also expresses the oil secretions from the meibomian glands. The lower lid moves horizontally in the nasal direction and pushes debris toward both punctae, the openings that ultimately drain into the nasal cavities.
As the eyelids open the tear film is redistributed where the upper lid pulls the aqueous phase via capillary action and the lipid layer spreads as quickly as the eyelids move. Hence, eyelid movement is accordingly important in tear-film renewal, distribution, turnover, and drainage.
For a variety of reasons, the meibomian glands can become blocked, plugged, or occluded resulting in meibomian gland dysfunction and dry eye disease. The obstruction that triggers the disease can occur anywhere within the meibomian gland, for instance, at the gland's surface or orifice preventing normal lipid secretions from flowing; in the main channel of the gland which may be narrowed or blocked; or in other locations deeper within the gland that lead to the main channel.
Treatments for blocked meibomian glands may include a number of conventional treatments. One course of treatment includes the application of soap and cleaning agents, eyelid scrubs, or antibiotics to reduce eyelid inflammation. Antibiotics such as tetracycline, doxycycline, metronidazole, or erythromycin can be administered orally or topically to help regulate or improve meibomian gland lipid production. Inflammation on the surface of the eye may also be controlled with topical drugs such as corticosteroids or cyclosporine (RESTASIS®, Allergan, Inc., CA), or other anti-inflammatory compounds or immune-suppressants. Evidence suggests that ocular surface inflammation is not only associated with meibomian gland dysfunction but also with dry eye syndrome.
Other examples of dry eye treatments may include the application of prescription eye inserts for people with moderate to severe dry eyes symptoms who are unable to use artificial tears. An eye insert, e.g., hydroxypropyl cellulose (LACRISERT®, Merck & Co., Inc., NJ), may be inserted between the lower eyelid and eye. The insert dissolves slowly to release a substance which lubricates the eye. Alternatively, special contact lenses may be used to shield the surface of the eye to trap moisture.
In other treatments, the patient's tear ducts may be closed to prevent the tear film from draining away from the surface of the eye too quickly by procedures such as insertion of punctal plugs into the tear ducts or cauterizing the tissues of the drainage area. Aside from implants or cauterizing treatments, dry eye syndrome may be treated using pharmaceutical agents such as eyedrops, ointments which coat the eyes, etc. Artificial tears, gels, ointments, autologous serum tears, or albumin drops have all been employed in the treatment of dry eye.
Additionally, warm compresses are also typically placed over the eyes and are used to restore function to the meibomian glands by melting any lipid plugs as well as incorporating massaging of the lids which may further express meibomian gland contents. However, application of warm compresses require their application two to three times daily during which time patients may incorrectly target only one of the affected lids and are also prevented from seeing out of the treated eye because of the compresses. Compresses may be too hot, further exacerbating inflammation, or they may cool too quickly preventing adequate therapeutic effect.
Other treatment devices have also been developed which cover the entire affected eye to apply heat and a massaging force directly to the affected eyelids. However, such devices, like the compresses, require that the patient's eyes be temporarily but completely obstructed during the treatment resulting in discomfort, lost productivity, and potentially lower compliance among patients. Additionally, these treatments require visits to a physician or healthcare provider and are not as well-suited for widespread consumer adoption.
Accordingly, there exists a need for methods and apparatus which are relatively simple to routinely use for the patient, which also allow for the patient to continue their normal activities, is non-obtrusive and non-disruptive, and which also take advantage of the patient's natural physiological activities to facilitate treatment.